# Thread: Surf Fisherman needs help w/ Aerodynamics

1. I am a surf fisherman and enjoy casting sinkers as far as possible. Currently 800 feet is my maximum distance.

Recently, I have been creating sinkers. I have also been reading about aerodynamics. They spend alot of time on airplanes, bullets and rockets, and of course cars, but I am interested in sinkers. I stumbled across a complicated explanation of why a parabolic shaped nose is best for subsonic flight, having to do with how fluid air moves around an object, but....subsonic comprises alot of different speeds.

I estimate my sinkers leave me at about 200 mph, they weigh 4 ounces, their angle of ascent is close to 45 degrees, and now I wish to reduce their drag. Reducing the drag is where I believe my attention should be focused.

What overall shape would you choose, if you were going to help me create a sinker with the least amount of drag, and get me past 800 feet ?

Pictures of current designs are at my photobucket site, as I am unable to post them here.
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2.

3. To attain a range of 800 feet, I estimate your sinker's initial velocity of at least 110 mph (Mach 0.14), probably a bit higher, so it's low subsonic.

All rear section shapes should end in a pointed taper, just like any aircraft wing, regardless of being supersonic or subsonic. The nose is a different matter. Pointed tapers work best at high speeds (high subsonic and supersonic). You're right in that rounded noses, such as parabolas, work best at subsonic -- the speeds at which your sinkers travel.

A parabola expands continually, and if it is simply fixed onto a cylinder, it will create a "shoulder" (ie, a 'discontinuity") that creates some extra drag (how much, I don't know). So, if you use a parabolic shape, you may want to round the shoulder.

I am familiar with the ogive of rifle bullets, etc. So for the sake of its simple manufacture, there's the idea of using a double-ended ogive (ie, the entire sinker is an ogive ... a circular arc rotated about the sinker's longitudinal axis {ie, like an American football except pointed and maybe not so "fat"}). Of course, it works most efficiently when pointed in the direction of travel instead of tumbling, so I suggest a few fins on its back end (like an arrow).

If you want to imitate the roundness of a parabola at the nose, you can trim/round a part of the nose's ogive into a spherical shape that joins the ogive tangentially. If you feel adventurous enough, you can try making the front part of the sinker an elliptical ogive (an ellipse rotated about its major axis). In either case, it would be similar to a typical symmetrical subsonic wing cross section, shown here.

In all cases, keep the ogive taper of the rear section to minimize the "base drag" (ie, the partial vacuum behind the projectile that pulls it backwards). A doubly-curved rear section (an ogive curving inward part of the way followed by an "anti-ogive" (of opposite curvature) that narrows to a pinpoint instead of an ogive's cone) might work a little better, but it doesn't seem worth the bother.

BTW, you might consider using a simple spherical (or football-shaped with fins) sinker launched from a slingshot. You can probably launch it with much more force and have greater control over its launch elevation and bearing. (Make sure to fasten the rod very securely to the ground so you don't lose it!)

4. Dimples make golf balls fly farther. Wouldn't the same be true of a lead sinker?

5. The use of all sorts of sports balls (ping pong, billiards, tennis, baseball, soccer, golf, etc) occupy a unique niche in the field of aerodynamics. These balls are spherical, of course, and experience a launch followed by ballistic flight. Most importantly, the players who launch these balls impart a spin on them to influence their ballistic flight. To affect its ballistic flight, the ball must "grip" the air. Dimples and other surface imperfections perform this "gripping".

Baseball particularly explains the physics of farther flight. We hear about baseball pitchers purposely scuffing up a ball, which already contain stitches (ie, surface imperfections). Farther flight can result from a couple of factors: keeping the ball in the air longer (time-wise), propelling it faster, decreasing the friction of the air on the object, etc. Baseball stitches (and golf ball dimples) keep the ball in the air longer by providing the ball with lift caused by surface imperfections on the spinning ball. The fast ball is thrown with the index and middle fingers on the top of the ball, and the ball rolls off those fingers, giving it a backwards spin (ie, relatively speaking, the top rolls backwards). I won't get into the aerodynamics of it now (it's called the Magnus Effect), but this surface motion results in lift. Pitchers actually throw the ball toward the ground, knowing that the lift will cause the ball to rise. Curve balls and sinker balls are also thrown with spins to cause them to move in those directions. Players have criticized the balls used in soccer's 2010 FIFA World Cup as having a diminished Magnus Effect and, thus, players cannot "bend it like Beckham" as much.

So, a fisherman would need to impart a particular spin (ie, like a fast ball) on a spherical sinker, which would also not have as much effect on it because of its high density (sinker are usually made of lead) and, thus, heavier.

6. FishinMortician was asking about reducing drag. Dimples increase trajectory of a ball by reducing drag and the ball doesn't have to be spinning for this drag reduction to occur. Spin can make the ball fly farther still, but spin is not required to achieve at least some of the benefit of dimples. That's my current understanding; happy to be corrected if wrong.

7. Dimples are effective on spherical shapes but not so much on streamlined shapes, as explained here. http://www.aerospaceweb.org/question...s/q0215.shtml0
The reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.

8. Yes. I dug around and found this, and it's a bit complicated. Thanks, Bunbury. The drawing above shows the reduction due to dimples of the drag-causing turbulence behind the ball at most speeds that a golf ball travels. A golf ball generally travels between 20 to 70 m/s. The pressure drag rises with ball speed, with dimpled balls generally having a higher drag than smooth balls. However, ...

All balls reach a critical speed below which the drag increases to a higher amount. For dimpled balls, this increase occurs at about 25 m/s; for smooth balls, it occurs at about 60 /ms. So, for two balls (smooth and dimpled) that begin at 70 m/s, the smooth ball has the advantage of a lower drag. As the balls slow below about 60 m/s, the smooth ball's drag transitions upward past the dimpled ball's drag. Then, until the balls slow to about 25 m/s (ie, most of the flight), the dimpled ball has the advantage. Then, about 25 m/s, the dimpled ball's drag transitions upward, and the smooth ball has the advantage again.

(I have a diagram of this but can't find a way to post it.)

9. Thank you for your kind responses. This is a wonderful forum.

My research and your explainations have allowed me to understand the aerodynamics of sinker shape. Very nice.

Understanding the perfect shape does not actually solve anything for me though. This because I have come to realize that the sinker is very unusual. It does not retain the same allignment during the entire flight. It must switch ends, back to front. This is because it goes from being pulled by the string (casting phase), to doing the pulling and bringing along the string (flight phase). Tricky.

The violence of this action imparts a wobble. The double ogive, if symetrical, would wobble like a rocking chair. Fins add drag and take time to force an alignment. A weight forward bias is probably the best solution. Being as the sinker is cast from lead, this means that the two ogives should not be of equal shape. This will create a shoulder, where dimples might indeed make the air transition more efficiently.

But....dimples ain't easy to do with moulds, and it would be very difficult for me to get them properly spaced and alligned.

I have given up.

I would however like to receive opinions on the design I am currently using. I have tried to post the picture, but all I can offer is a place to go to see it.

http://i267.photobucket.com/albums/i...ian/002-11.jpg

This sinker is one I concieved, designed, and formed. It is a fishing sinker and not an absolute distance sinker. It has a powder coated head to eliminate lead transfer when manipulating it. The blue assembly allows the grip wires to engage the sand for grip in the current, and then release when a fish pulls against them. They simply pivot backwards and disengage. Differing tensions are adjusted at the assembly. There is a bait clip (small hook shaped wire) that is used to hold the baited hook in behind the sinker, within the slip stream, and this allows the trailing rig with it's components to remain streamlined.

Is there much of anything that can be done to this sinker to improve it's performance ?

10. it looks like you are on the right track.........

look into the Coandă effect, generally in aerodynamics you want an ogive front, attached laminar flow and a 'clean' tail.

apart from the front (which you have already done) the big issue is always the rear - it has to be much larger and longer than the front as the fluid (be it air or whatever) does not like to go round any corners or sharp inclines. in this respect the front is easy as it can be VERY curved and is resultantly space efficient. the front acts like the top of an aircraft wing so the faster the air flows the more lift is generated in the forward direction.

lift is still a form of drag (it is all drag really) just a more effective type if used in the right way.

useful video below - there are loads on you tube -